Antigen Receptors on Lymphocytes

Year: 1986 DOI: 10.7907/p1zd-zh51
The structure and evolution of a small VH gene family called the T15 family was analyzed. It was determined that although selection pressure appeared to be operating to maintain the coding region sequence of these VH gene segments, these gene segments were diverging from one another very rapidly. Sequences were identified in the 5' flanking region that were conserved between all VH gene segments and were hypothesized to be important for immunoglobulin heavy-chain gene transcription. Related sequences were identified in immunoglobulin VL gene segment and histone H2B 5' flanking regions, implying coordinate expression between these genes and the immunoglobulin heavy chain genes.

The structure, organization, evolution, and the generation of diversity in the genes encoding the T-cell antigen receptor were analyzed. The T-cell antigen receptor consists of two chains, referred to as the α and β chains. Each chain consists of two regions, a variable region and a constant region, that are encoded by two different genes. The gene that encodes the variable region of the β chain was found to consist of three gene segments, denoted Vβ, Dβ, and Jβ. The Vβ gene segment encodes the first 280-300 bp, the Dβ gene segment encodes the next 10-15 bp, and the Jβ gene segment encodes the final 50 bp of the variable region gene. The rearrangement event that juxtaposes these gene segments during lymphocyte differentiation appears to be mediated by the same recognition signals that mediate immunoglobulin V gene rearrangement.

Diversity was found to be generated in at least three different manners in the Vβ gene. Combinatorial joining permits the rearrangement of different V, D and J gene segments to each other to provide different V gene sequences. Deletion of nucleotides from the ends of the germline gene segments and the random addition of nucleotides at the junction of the rearrangement event are two other mechanisms for generating diversity. A comparison of a rearranged V gene with the corresponding germline gene segments showed that with the exception of the junctions, the sequences were identical. Therefore, there is no evidence that somatic hypermutation, the random addition of point mutations to the V gene during late stages of B lymphocyte differentiation, is utilized by the T-cell antigen receptor as it is by immunoglobulins.

The initial stage of Vβ gene formation was found to be the rearrangement of the Dβ gene segment to the Jβ gene segment. Both germ line Dβ gene segments appear to have promoters in the 5' flanking regions that can often result in the transcription of a 1.0 kb mRNA containing Dβ-Jβ-Cβ sequences after Dβ-Jβ rearrangement. This 1.0 kb mRNA message is present at a high level in the thymus but at lower levels in the spleen, lymph nodes, and in mature T cells, implying that this message or a protein product encoded by this message may be important in T cell ontogeny.

Analysis of the protein sequences of the variable regions of the α and β chains revealed conserved amino acids that are found in all variable region genes. Many of these amino acids were found to be important for V domain structure in immunoglobulins and may be important for the structure of the Vα-Vβ domain as well. In addition, analyses of the β-strand forming potential and the relative hydrophobicity of the side chains of the amino acids that make up the T-cell antigen receptor variable regions have indicated that these properties are very similar to those of the immunoglobulin variable regions. These analyses indicate that the immunoglobulin and T-cell receptor antigen-binding regions may be very similar in structure to each other.